The transient spedtrometer has been further developed and used to measure the spectral evoluation of deoxyhemoglobin produced by photolysis of carbonmonoxyhemoglobin. Several temporally distinct processes are observed. The first product exhibits a weak, blue-shifted spectrum peaking at 423 nm and decays within about 5-10 nanoseconds to a broadened spectrum peaking at 430 nm. Photodissociated carbon monoxide recombines with this species with an apparent rate of 1.5 (10) sec which is independent of the free CO concentration. This process is interpreted as recombination of CO which is dissociated from the heme iron, but which remains trapped in the surrounding protein matrix. Subsequent to this 'geminate' recombination, a spectral change characteristic of the hemoglobin quarternary structural change can be resolved. Systematic observations of these processes as a various temperatures and CO pressures have been initiated in an effort to unravel the origin of the spectral changes and the mechanism of the geminate recombination.